Various magnetic recording techniques exist for recording data to and from magnetic storage media, such as magnetic tape. Magnetic tapes are used for data storage in computer systems requiring data removability, low-cost data storage, high data-rate capability, and high volumetric efficiency and reusability. The rapidly accelerating growth in stored digital data and images, the Internet, and replacement of paper as long-term record retention, and the need for massive dense storage for reconnaissance and surveillance is creating a demand for corresponding increases in the data storage capacities of magnetic tape recording and reproducing systems, while maintaining the special requirements of high speed digital tape systems.
Tape recording and reproducing systems for use as computer data storage devices are often required to provide high data transfer rates and to perform a read check on all written data. To satisfy these requirements, conventional or orthogonal linear tape systems (where recorded transition lines that are created between regions of opposite magnetization are orthogonal to the head/tape motion direction) typically employ methods wherein the tracks of data lie parallel to each other and to the edge of the tape. Linear recording techniques offer high data transfer rates by employing reading and writing head configurations with multiple parallel channels, wherein each read and write head pair provides a channel with each writing or reading element in data transfer contact with the recording media 100% of the time.
In orthogonal linear tape recording systems, data tracks are followed in the direction of tape movement with the read and write heads arranged in the same manner as the recorded transitions that are perpendicular to the direction of tape motion. The write head defines the width of a data track (and thus the number of data tracks that can be provided across a tape of given width) by creating the regions or domains of magnetization following one another in the tape direction at the width of the write head. The potential for misregistration of the read element to the written track (from tape wander, data track alignment or the like) requires that the read head be substantially smaller than the written track width in order to ensure that the read head is reading magnetization fields only within the desired data track. Thus, the read head size (as is also limited by read head performance characteristics) limits how narrow the data track can be, hence the maximum track density. That is, not only is the data track width limited by the minimum read head size in order to meet the recording system's performance criterion, it must be sufficiently wider to accommodate expected misregistration as may occur under the dynamic conditions of moving media and as may be determined empirically or by modeling. If a read head moves off the data track for whatever reason and begins to read a signal from the adjacent track, the possibility of erroneous data transfer increases. More specifically, the error rate is known to increase exponentially as the read head moves further off the data track. Typically, for an acceptable off-track error rate, the adjacent track signal must be less than ten percent of the data track signal. The general premise is thus to write wide and read narrow. Writing wide, however, decreases the data density (less data tracks across a given tape width). Reading narrow is unfortunately limited by making an acceptable read head that will still meet signal amplitude, SNR, and media defect sensitivity requirements. As a result, minimum track width is approximately the width of a read element that meets the above performance requirements plus twice the misregistration (normally the three sigma value since the misregistration is a statistical distribution).
There are a number of potential sources of read element to written track misregistration error, which error is systematic in that both the media and the drive are involved as potential sources of error. The principal sources of error are: tape lateral motion, vibration in the head/actuator assembly, dimensional instability of the media substrate, and mechanical misalignments between read and write elements in manufacturing and assembly. Probably the most significant limitation on tape track densities is the tendency for the tape to experience lateral tape motion, which is a tendency for the tape to shift laterally relative to the linear direction of tape motion. During a data track write, lateral tape motion can cause one or more data tracks to deviate from a desired axis along which tracks are expected to be written. During reading, lateral tape motion can cause misregistration of the read head over the track being read. This build-up of potential misregistration of data tracks combined with other less significant potential sources of misregistration can result in a portion of the read element to be positioned over an adjacent data track, which, if significant enough, can cause an unacceptable level of data transfer errors. As noted above, the reading of an adjacent track is typically limited to ten percent or less of the desired data track signal. The normal method in linear tape recording to ameliorate the potential effects of this misregistration is to make the read element much narrower, i.e., approximately half, than the track width. However, as noted above, limitations of minimum signal amplitude, signal-to-noise-ratio, and sensitivity to media defects provides a lower limit as to how narrow the read element can actually be. Thus, from a practical design stand point, an effective read head size as determined by such performance constraints would be doubled to determine a desirable data track width. As such, the effective read head size limits how narrow a data track can be made.
One developed method of increasing track density involves azimuth recording techniques. Azimuth recording has long been used in helical recording systems and has been more recently introduced into linear tape systems. Generally, in azimuth recording of either helical or linear tape systems, data transitions on alternate adjacent tracks are recorded at a same but opposite angle relative to an axis along which the head travels relative to the media. In helical tape recording systems, the head is moved relative to a linear tape movement at a significantly greater speed and at an angle to the direction of tape movement.
Azimuth recording itself is a well-understood technology that provides a level of suppression of an adjacent track signal. The suppression is based upon the well known relationship that the suppression, S=20*log10[sin x/x], where x=(πW/λ)*tan 2θ. In this relationship W is the data track width, θ is the angle that the recorded transitions make with the transverse axis to the head direction, and λ is the wavelength associated with the minimum transition density (λ=two times the maximum transition spacing). Thus, a determined azimuth angle, θ, is dependent on: the degree of suppression to be attained, the data track width W, and the minimum transition density or maximum λ of the readback signal spectra. In current systems the data track width W is at least an order of magnitude larger than λ and thus, a suitable transition angle θ can be relatively small to achieve sufficient suppression of an adjacent data track signal.
Because of such angular azimuth recording, a signal from a track adjacent to the data track being read can be sufficiently suppressed to an acceptable level, such as to be less than ten percent of the data track signal as noted above. Hence, a read element can overextend an adjacent track and thus can be designed to be wider than the data track, allowing the full data track signal to be utilized.
In FIG. 1, a section of magnetic tape 10 that includes data recorded by an azimuth recording technique is schematically illustrated. Three adjacent data tracks, 12, 14, and 16, that have track widths 13, 15, and 17 respectively, are schematically illustrated. The data tracks 12, 14, and 16 each include a plurality of data transitions, 18, 20, and 22, respectively, that are spaced apart from each other within each individual track to define a transition density (number of data transitions per unit length of track). The data transitions are provided and can be counted along a direction of travel 24 of the tape 10 that also includes a transverse axis 26 (an axis perpendicular to the travel direction of the tape). As illustrated, the data transitions (indicated by the lines that would be created between adjacent fields of opposite magnetization) are provided at an angle θ with respect to the transverse axis 26 and they are provided so that adjacent data transitions of adjacent tracks are provided at an angle to each other 2θ.
A read head 28 is shown positioned above the data track 14 and can be used for reading the data transitions 20 of the data track 14. As shown, the head 28 extends across the track 14 and also extends partially over adjacent tracks 12 and 16. Thus, when reading the data transitions 20, the head 28 will also read a portion of the signals from adjacent tracks 12 and 16. Because the data transitions 18 and 22 of the tracks 12 and 16, respectively, are provided at an angle to the data transitions of the track 14, the signal read from the adjacent tracks 12 and 16 can be at least partially suppressed based upon the relationship of suppression to track width W, wavelength λ, and transition angle θ identified above. From the sin x/x relationship of the equation above, an expression x at 90° would achieve the greatest suppression. The expression x, however, is determined in part by the relationship of the tangent of 2θ (two times the transition angle θ). Whereas the tangent approaches infinity as 2θ is increased to 90° and a greater value of tan 2θ increases the value of X, the suppression of an adjacent track signal would increase to a transition angle θ limit of 45 degrees. However, acceptable levels of suppression are attained in current azimuth recording systems at transition angles well below the maximum of 45 degrees. Importantly, there is also a well accepted design factor to achieve greater overall recording density (looking at tape surface area) that works against a desire to increase the transition angle θ at all, thus limiting a designer to not increase the transition angle θ beyond that which is necessary to effectively achieve an acceptable suppression of an adjacent track signal. That is, it has been determined that as the azimuth angle increases, the increase in the suppression per unit of angle increase diminishes and the minimum separation between transitions in the direction normal to the azimuth angle decreases resulting in the number of transitions per unit length of track decreasing by approximately cosine θ. Thus, under the current approach to azimuth recording, it is accepted that it is only desirable to increase the azimuth angle to achieve the desired suppression level in order to minimize the “cosine loss” as described in U.S. Pat. No. 4,539,615, “Azimuthal Magnetic Recording and Reproducing Apparatus.” The result is that for an acceptable level of suppression taken as a given along with track width and transition spacing, any further increase of the azimuth angle would result in a lowered overall recording density. Thus, current azimuth recording techniques limit azimuth angles of such transitions to less than about 20 degrees.
Current linear serpentine tape drives for azimuth recording typically utilize a single head structure that contains two pairs of read and write elements. Like orthogonal head structures, azimuthal head structures are typically designed with the read and write elements parallel to each other and aligned in the direction of tape movement. As such, the head is rotated to bring the appropriate read/write pair into proper alignment at the desired azimuth angle. One limitation of using a single head is that two degrees of freedom of movement, e.g. rotation and transverse shifting of the head, are required for track accessing and track following. The mechanical joining of the read and write elements makes it difficult to achieve independent tracking of the read and write elements without such multiple freedoms of movement.
By offsetting the read and write elements as they are positioned along lines that are parallel to one another as to the distance along the parallel lines, an orthogonally constructed head can be positioned to record and read azimuthal tracks when rotated at an appropriate angle. The read and write elements can be aligned so that with the proper spatial relationship between them, they are able to read and write adjacent tracks and only require transversal repositioning once for every track pair. Such transversal movement and positioning or tracking can be conventionally controlled by known actuators. Tracking can be achieved in a single head, but usually requires the additional complexity and weight of a dual degree freedom actuator, such as conventionally known and that permits both rotary movement of the single head and movement of the head in the transverse direction to the tape movement. A compound dual degree freedom of motion actuator, i.e. a single unit to provide multiple types of motion, adds additional mass and generally needs to carry twice as many leads in order to accommodate forward and reverse read and write capabilities. This provision of additional leads adds stiffness to the system that can inhibit or interfere with its motion capabilities.
Recent generations of multi-channel linear serpentine tape systems have used servo tracking to decrease track misregistration. The use of servo tracking has greatly reduced tracking errors due to manufacturing alignment and offset tolerances between the read and write element arrays, skew errors, some track shift due to tape substrate dimensional instability, and the effect of lateral tape motion. In such systems, position sensing read sensors (servo elements) detect prewritten servo tracks on the tape that can be laid down under more tightly controlled conditions to reduce misalignment of the servo tracks to the tape. The tape is typically divided into alternating bands of data tracks and servo tracks where the band of data tracks can be much wider than the servo band; typically the data band is 8 to 16 times the width of the servo band, depending on the number of data channels. From the output signals of the servo data elements, a position error signal can be determined that is used by the servo control loop to dynamically and more accurately position the data elements over their tracks. Typically, the servo elements are located in the same array as the read elements and are symmetrically placed outboard of the read array on each side.
Examples of typical primary head configurations for prior art orthogonal and azimuthal recording systems that utilize parallel gap lines on linear tape are shown in FIGS. 3-5. A common feature of the three illustrated configurations of FIGS. 3-5 is the inclusion of parallel read and write gaplines. Also basically common to all three configurations is the read and write modules, which differ in the thin film structure that is structured in known ways depending on whether the thin film element is part of a read or write module. In FIG. 2, a basic read or write module 30 that may be used in the prior art configurations or in making up modules in accordance with the present invention comprises a substrate 31 that is typically ceramic upon which is deposited a multi-layer patterned thin film sandwich 32 (structured as a read or write element) and that forms a gapline 33 that itself typically comprises a material layered onto the surface of the substrate 31 adjacent to the thin film structure 32, but may comprise any material effective for defining an operative magnetic gap. A closure piece 34, also typically ceramic, is shown that closes the gapline 33. The closure piece 34 would typically be bonded or otherwise connected in place to the ceramic substrate 31 aside or below the provision of the gapline 33 and/or directly to the material defining the gapline 33. After assembly, the module 30 would be conventionally machined and lapped to provide a tape bearing surface 35. The substrate 31 and closure 34 are normally comprised of the same ceramic material which may have several of the following characteristics: the material may be magnetic or non-magnetic and it maybe non-conductive, resistive, or conductive.
A standard orthogonal arrangement of heads for bi-directional tape motion is shown in FIG. 3. A portion of the magnetic tape 40 is illustrated, and in particular two adjacent data tracks 41 and 43 of any number of such data tracks are shown without transition lines to illustrate the orientation of head modules 50 and 55. When the tape 40 moves in a forward direction 42 (i.e. from the beginning of tape), the track 41, as an example, can be in a data transfer relationship with the head modules 50 and 55. When the tape 40 and thus track 43 moves in the reverse direction 44 (i.e. from the end of tape) and when the head modules 50 and 55 are shifted in the transverse direction 47 over the data track 43, then data transfer can be established between the head modules 50 and 55 with the data track 43. It is necessary during a read-while-write function (data writing followed by confirmation read) that the writing and reading are accomplished in separate gaplines because of the high level of interference that may be created as a result of the relatively much larger write signal coupling into the read signal, which interference can still occur if the write element (or array) and read element (or array) are in too close of a proximity to one another. The closer the proximity of the two gaplines, the larger is the potential for interference.
In the orientation shown, a first head module 50 comprises a write element 51 and a read element 52 that are positioned for writing followed by reading track 41 in the forward direction 42. A second head module 55 comprises a write element 57 and a read element 56 that are shown in FIG. 3 also positioned over track 41, but which can be shifted transversely as indicated by arrow 47, so as to be positioned for writing followed by reading of track 43 in the reverse direction 44. The two head modules can be bonded together to provide a unitary head module structure at interfaces 58 and 59, and with a construction as illustrated, would have the read and write gaplines aligned and parallel to one another in a direction perpendicular to the direction of tape motions 42 and 44. However, this system is limited in the ability to precisely position the read and write elements since the two head modules form a monolithic head and the read and write gaplines cannot be independently positioned. Alternatively, the head modules 50 and 55 can be independently controlled by the additional complexity of duplicate separately provided mechanisms for movement. One common configuration for independent precision positioning of the read and write elements utilizes both head modules 50 and 55 in each direction. So, for tape movement in the forward direction 42, writing is conducted by write element 51 of head module 50 and reading is conducted by read element 57 of head module 55. Conversely, writing is conducted by write element 56 of head module 55 and reading is conducted by read element 52 of head module 50 for tape movement in the reverse direction 44.
A dual independent head configuration for azimuth recording on linear tape is shown in FIG. 4. Two adjacent tracks 61 and 63 are shown with angled transitions as would be recorded on tape 60 with track 61 written in the forward direction 62 and track 63 written in the reverse direction 64. The transitions of track 61 are illustrated as being written at an azimuth angle θ by the write element 71 (or array of such elements) on head module 70 and read by read element 72 (or array of such elements) on head module 70. Likewise, track 63 is illustrated as being written at a similar azimuth angle of −θ by write element 77 (or array of such elements) on head module 75 and read by read element 76 (or array of such elements) on head module 75. For maximum precision in positioning head modules 70 and 75, independently positionable actuating mechanisms having two degrees of freedom allow the read and write element arrays in each head module to be positioned with respect to each other. Typically, the actuating mechanisms are actually connected together by a fixed mechanical linkage so that the positioning of one array influences the positioning of the other array; hence, any control system must be constructed to also accommodate these effects. For operation, the azimuthal head modules 70 and 75 move transversely in direction 67 in a similar manner as that described above for an orthogonal system with each head module responsible for data transfer with one of the tracks that, in this case, have oppositely angled transitions. Although the transitions of one track 61 need not be at the same angle θ as the transitions of an adjacent track 63, it is normal that they are to achieve similar recording densities.
FIG. 5 illustrates the use of a single head 90 that is rotatably positionable to provide a similar data transfer relationship between the head 90 and either of the oppositely arranged data tracks 81 and 83. The head 90 is also movable in transverse direction 87 to provide access to any number of such data tracks across the width of tape 80. The head 90 can be rotated through an angle of 2θ to achieve an azimuth angle of θ for track 81 when writing or reading in the forward direction 82 and for writing and reading at a similar azimuth angle of −θ for data transfer with track 83 in the reverse direction. The head 90 includes three modules comprising a write element that is selectively positionable at positions 92 and 92′ for writing in either direction 82 or 84 and a pair of read elements, one being positionable at 93 for reading track 81 in the forward direction 82 and the other read element being positionable at 91′ for reading track 83 in the reverse direction 84. The read element positions noted at 91 and 93′ are not used for data transfer with either track 81 or 83. A complementary configuration where a plurality of write elements are used with one read module is also possible and may be arranged instead with a read element positionable at 92 and 92′. Again, for precise positioning a two degree of freedom actuator is required to quasi-independently position the read and write elements.